1. Field of the Invention
The present invention relates to a semiconductor device including a storage device including a memory element utilizing silicide reaction, and a method for driving the same.
2. Description of the Related Art
In modern society, where many electronic appliances are used, various data are generated and used; therefore, storage devices are required to store the data. Various storage devices produced and used today have different advantages and disadvantages, and are used properly depending on the data to be stored and used.
For example, volatile memory that loses its memory content when the power is turned off includes DRAM and SRAM. The applications of volatile memory are significantly limited because memory content is lost when the power is turned off; however, each of them is used as a main storage device or cash memory of a computer taking advantage of a short access time. Since DRAM has small memory cells, large-capacity DRAM can be produced easily. However, it is controlled in a complex manner and consumes much power. SRAM includes a memory cell formed from a CMOS and is easily manufactured and controlled; however, it is difficult to obtain large-capacity SRAM since one memory cell needs six transistors.
Nonvolatile memory that holds its memory content even after the power is turned off includes: rewritable memory into which data can be rewritten many times; write-once memory into which data can be written by a user only once; mask ROM of which data content determined in the manufacturing of the memory cannot be rewritten; and the like. As the rewritable memory, there are EPROM, flash memory, ferroelectric memory, and the like. The EPROM allows an easy writing of data and unit cost per bit is relatively low; however, a program device and an eraser dedicated to writing and erasing are required. The flash memory and the ferroelectric memory can be rewritten on a substrate used, have a short access time, and consume less power; however, steps for manufacturing a floating gate and a ferroelectric layer are required. Thus, the unit cost per bit is high.
Each write-once memory includes a fuse, an antifuse, a cross pointer diode, an OLED (organic light emitting diode), a bistable liquid crystal element, and other devices whose states are changed by heat or light. Further, in recent years, memory elements using silicide reaction are also developed (for example, Reference 1: Japanese Patent No. 3501416). Memory described in Reference 1 includes a plurality of memory elements formed using conductive films serving as a cathode and an anode and an amorphous silicon film provided between the cathode and the anode.
As a method for writing data into memory, a method for applying electrical action is proposed. In the case where data is written by electrical action, an element with application of high voltage between a pair of electrodes to make an amorphous silicon film therein silicided and an element with no application of high voltage between a pair of electrodes to make an amorphous silicon film therein silicided are manufactured, and data is written into the memory. Then, voltage is applied to the memory elements for reading and a difference in resistance between the memory elements are read, whereby data of “0” and data of “1” can be distinguished from each other.
However, in the memory into which data is written by electrical action, both the memory element in which the silicon film is silicided and the memory element in which the silicon film is not silicided exist in the memory. Accordingly, when the same voltage as that applied during writing is applied for reading, the silicon film of the memory element which has not been silicided is also silicided; therefore, it is necessary to change voltage value for the memory elements for reading to be different from voltage value for writing. For example, provided that writing voltage is X [V], it is necessary to apply lower voltage than X for reading in order to prevent the silicon film of the memory element in which writing is not performed, from being changed by silicide reaction. In general, as an external power supply of a memory, an external power supply is used, from which a voltage value used for reading data written into a memory element is output. Accordingly, for writing data into the memory element, voltage necessary for writing is obtained by boosting the external power supply voltage with the use of a boosting circuit.
A generally-used circuit for changing a voltage value, such as a boosting circuit, is large in volume. Therefore, it is difficult to miniaturize memory in which a circuit for changing a voltage value is needed.